The invention relates to a method for the formation of a frangible zone for fracture separation of a part, in particular, connecting rods for internal combustion machines. A frangible zone is produced for the intended triggered fracture having ribs and depressions which are separated, one from the other, and arranged at least partially in the surface of the part along one side of the fracture separation plane.
A method for the formation of stress riser grooves in the fracture plane of a separable part is known through the U.S. Pat. No. 5,208,979. To produce stress riser grooves that are essentially knife-sharp at their bases, grooves having a V-shaped cross section are formed in the part's surface by means of a laser at appropriate locations in or on the part. The depth of the groove is greater than its width. As is known, the hairline cracks that initially form at the beginning of a fracturing process at the bottom of a stress riser groove are of considerable importance for the quality of the fracturing surfaces, said cracks not being deliberately predictable because of the inhomogeneous structure of the part over the length of the stress riser grooves.
This disadvantage should be overcome in accordance with German patent document DE-U 295 19 126 in the case of a formed workpiece, especially a connecting rod with an opening (a bore for a bearing), having diametrically opposed frangible zones. Instead of every continuous, straight line V-groove being produced by a laser in accord with U.S. Pat. No. 5,208,979, a plurality of indentations laid linearly next to each other serve as groove sections. The laser employed for the boring of the groove sections into finger shaped, or cylindrical blind holes, is so far reduced in load capacity by this procedure, that the transformation associated with heat input (Martinsite) in the adjacent areas of the groove sections is exceptionally small. It is a stated goal, to avoid in all cases, a continuous transformation occurring in the ribs which remain between the groove sections, in order that these frangible zones can be machined away following a fracture.
Further, the production of a series of blind holes, in a naturally hardened work piece next to one another by means of a laser for the purpose of engendering a controlled fracture, is also known, as this is described in the American magazine "Mechanical Engineering" in the April issue of 1990 in the article "Detroit looks to lasers" on page 41 in the left column.
The goal of the invention is to form the intended frangible zones in a fracturing plane for the predicted production of a crack, in a part made of ductile metal and divisible by fracturing in such a manner that starting cracks are achieved that are deliberately distributed over the length of the respective frangible zone.
The present invention achieves this goal by a method for the formation of a frangible zone for fracture separation of a part, in particular, connecting rods for internal combustion machines. A frangible zone is produced for the intended triggered fracture having ribs and depressions which are separated, one from the other, and arranged at least partially in the surface of the part along one side of the fracture separation plane. In the part, such as a rod, made of ductile metal, the ribs, at least in their free end sections near to the surface of the part, are penetratively embrittled for making an initial starting crack for triggering the break. This is done by performing a transformation across their cross sections. In the case of a part of ductile metal, the ribs, at least in their free end segments near to the surface of the part, become penetratably embrittled by a material transformation over their cross section, thus making the initial cracking for the triggering of the fracture.
Along with the frangible zones produced in accordance with the invention, contrary to a conventional, continuous groove parting line, or contrary to groove sections of softened material arranged in rows, there arises in an advantageous manner a number of transversely embrittled ribs defining short line segments, the length of which in total is substantially shorter, as opposed to the continuous groove separating line or a plurality of groove portions. By means of this situation, the fracturing severance strength, apportioned over a small cross section in the embrittled break-susceptible ribs, acts over the frangible zone to trigger the initiation of cracking. With the choice of the rib cross section in combination with the respective embrittlement process, the simultaneous triggering of starting cracks over the entire, invention oriented, frangible zone is achieved in an advantageous manner. Thereby, a point to point starting crack is avoided. Starting cracks per se, in sinter forged parts, among others, are made known by the German patent document DE-C 38 06 236. In the formulation of the invention, this advantage is thereby enhanced, in that the depressions in the respective base between the ribs, at least in the area of the fracture plane, are additionally embrittled.
Out of this arises the advantage that in the respectively weakest cross section of the rib, self elongating starting cracks combine with each other in reaching the respective base between the depressions to form a continuing fracture front as they progress through the rib.
Without expensive measures, the fracturing points in accordance with the invention in the case of a part made of a ferrous material with a carbon content of C.apprxeq.0.55 to 0.85% can be made, since the embrittlement of the ribs and/or the depressions are achievable by temperature dependent transformation. Especially advantageous is a hardening with self quenching which serves as a means of engendering transformation.
This measure is especially advantageously carried out, in that the depressions are created in the course of the procedure in accordance with the invention, that is, on a known basis of the fusing of the material of the part through radiant energy. The radiant energy is so applied by a laser, and the fused material is ejected or burned by means of a jet of air or oxygen. As a result, at least in the free end segments of the ribs, close to the surface of the part, the initial starting cracks are produced in embrittled transformed zones which overlap themselves at an acute angle. For the avoidance of subsequent finishing work in the area of the frangible zone, the ribs between neighboring depressions are disposed deeper, or deepened in relation to the surface of the part and, advantageously, the air or oxygen jet additionally serves this end.
The overlapped, embrittled zones produced by the procedure in accordance with the invention, are created in an especially advantageous and simple manner. The depressions are formed in a conical shape by a laser apparatus with a beam energy divided over the cross section of the pulsed laser beam according to the Gaussian distribution curve.
Through this clever application of the laser for the formation of the depressions, the goal is reached that a generally glass hard coating on all sides of the depressions formed through self quenching also permeates the ribs in the smallest cross sections. What is further achieved thereby, is a formation which is fracture sensitive and reaches to the very base of the frangible zone. This formation, depending upon the hardening process, can immediately exhibit hardening cracks as starting cracks.
The depressions formed advantageously by a pulsed laser show in respective optional cross section in the direction of their arrangement beside one another on the surface of the part a breadth of 0.4 mm and a depth between about 0.4 to 0.9 mm. The advantage of these relatively narrow frangible zones, is that they can remain, depending on the finishing operation of a bearing boring, with additional installation of bearing bushings or roller bearings to the part. Support for this is provided according to a further feature of the procedure in accordance with the invention, in that the embrittled rib tips produced from the top cut-off portions of neighboring, conically formed depressions are permitted to remain unchanged in their respective intervals deepened in relation to the surface of the part.
For carrying out the procedure in accordance with the invention, in further embodiments, an apparatus of advantageously simple construction is proposed, in which the laser devices are employed in an opening (bearing bore) of a part with interpenetrating, crossing laser beams in simultaneous, diametrically opposed frangible zones, whereby for the protection of the laser optical system, the part in a treatment position relative to the fixed laser equipment is moved forward for sequential formation of the depressions which are arranged-in an adjacently spaced manner.
Within the framework of the invention there is also known another type of ribs and depressions, which are annealed by means of an electron beam to achieve an increased fracturing sensitivity as is taught by U.S. Pat. No. 3,818,577.
Further, embrittlement is offered by means of thermal processes, mechanical working, and/or by means of cast ribs, and/or depressions by means of the occlusion of a gas in the metal of the respective part, as this is especially made known in the U.S. Pat. No. 4,970,783 which describes hydrogen embrittlement.
In an embodiment of a part having diametrically opposed frangible zones in a bearing boring, the procedure in accordance with the invention finds a preferential application in that, first the frangible zones are installed prior to the final finishing operation of the respective bearing boring of a formed connecting rod or machine bearing part, and second, the respective bearing cap between cap screws is so flexibly made that, with a broach chosen 0.05 to 0.15 mm smaller in diameter relative to the premachined bearing boring, an elastic deformation ensues that serves to create starting cracks in the ribs and/or the depressions. The advantage of this measure is that high tension stress is provided at the start of the fracture loading in the embrittled ribs for the triggering of starting cracks in which high quality break surfaces are of importance.